Perhaps the most heavily used equation in physics is Newton's second
law of motion:

That is, when a force is applied to a mass, the mass experiences an
acceleration proportional to the applied force. Denoting the mass by
, force at time
by
, and acceleration by

we have

(B.1)

In this formulation, the applied force
is considered positive
in the direction of positive mass-position
. The force
and acceleration
are, in general, vectors in
three-dimensional space
. In other words, force and
acceleration are generally vector-valued functions of time
. The
mass
is a scalar quantity, and can be considered a measure of the
inertia of the physical system (see §B.1.1 below).